With reduction in size and weight of electronic devices, electrical parts used in electronic devices, such as cells and capacitors, have also been required to be reduced in size and weight. Consequently, for example, a nonaqueous electrolyte cell has been employed in which a bag body is used as a sealable container, and a nonaqueous electrolyte (electrolytic solution), a positive electrode, and a negative electrode are enclosed therein. As the nonaqueous electrolyte, an electrolytic solution prepared by dissolving a fluorine-containing lithium salt, such as LiPF6 or LiBF4, in propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, or the like is used.
Such a sealable container is required to have properties of preventing permeation of an electrolytic solution and gas therethrough and infiltration of moisture from the outside. Therefore, a laminate film including a metal layer, such as an aluminum foil, coated with a resin is used as a material for a sealable container. By fusion-bonding the edges of two laminate films, a sealable container is formed.
An opening portion is provided on one end of the sealable container, and a nonaqueous electrolyte, a positive electrode plate, a negative electrode plate, a separator, and the like are enclosed therein. Furthermore, a lead conductor having one end thereof connected to the positive electrode plate and a lead conductor having one end thereof connected to the negative electrode plate are arranged so as to extend from the inside to the outside of the sealable container. Lastly, by heat-sealing (fusion-bonding) the opening portion, the opening portion of the sealable container is closed, and the sealable container and each of the lead conductors are bonded together to seal the opening portion. The portion that is heat-sealed last is referred to as a sealing portion.
In this process, the sealable container and each of the lead conductors are bonded (fusion-bonded) together through a thermal adhesive layer. The thermal adhesive layer is provided in advance on a part of the lead conductor corresponding to the sealing portion, or provided in advance on a part of the sealable container corresponding to the sealing portion so that it can be interposed between the metal layer of the sealable container and the lead conductor. When the thermal adhesive layer has high fluidity at the time of heat-sealing, adhesiveness between the sealable container and the lead conductor can be enhanced. However, when fluidity is excessively high at the time of fusion-bonding, short-circuiting may occur between the metal layer and the lead conductor. Therefore, the sealing portion is required to have a characteristic of being capable of maintaining adhesion and sealing properties without causing short-circuiting between the metal layer and the lead conductor.
Japanese Patent No. 3562129 discloses a sealing bag for a battery and a lead wire which are used for such a nonaqueous electrolyte cell, and describes that the sealing property of a sealing portion can be enhanced by providing a maleic modified polyolefin layer, which serves as a thermal adhesive layer, directly on the conductor of a lead conductor.